Transcript Slide 1

Developing the Atomic Theory
1
Lesson 5
October 1st, 2010
Evolving Models of the Atom
• An atom is the smallest part of an
element that has all of the element’s
properties.
• Different kinds of atoms give elements
different properties.
Evolving Models of the Atom
• Atomic theory is the study of the nature of
atoms and how they combine to form all
types of matter. Atomic theory helps us to
understand why there are different kinds of
atoms. It explains how atoms combine to
form over 100 known elements and all other
forms of matter, including compounds and
mixtures.
Evolving Models of the Atom
• Scientists have been wondering what matter
is made of for over 2500 years.
Evolving Models of the Atom
• Empedocles theorised that all substances
were made from the combination of four
fundamental elements: earth, air, fire,
water.
• Today, we still use the term element, though
in a different way. For example, we still
believe that most substances are built up
from simpler ones.
• About 440 B.C.E., the Greek philosopher
Democritus hypothesized that breaking
down rock into powder and then breaking
that into smaller and smaller pieces unit a
single invisible particle was reached. He
called this particle an Atom which means
uncutable or invisible in Greek
• Neither ancient scientist did any
experimentation when they developed these
models.
Atomic Theory Takes Shape
• Atomic theory is rooted in the idea that
an understanding of atoms and their
structure can help us predict many of the
properties of matter.
John Dalton (1766–1844),
• a scientist and teacher in England,
reconsidered the ancient idea that each
different kind of element is composed of a
different kind of atom. Dalton thought that
the atom would be like a featureless
sphere.
John Dalton (1766–1844),
He used the following theory to explain the
nature of matter:
• All matter is made of small, indivisible
particles called atoms.
• All the atoms of an element are identical in
properties such as size and mass.
John Dalton (1766–1844),
• Atoms of different elements have different
properties.
• Atoms of different elements can combine in
specific ways to form new substances.
• Atoms cannot be created, destroyed, or
subdivided in a chemical change.
• Dalton also devised a series
of element symbols to
represent the atoms of
different elements. These
symbols are shown in his
Table of Elements from
1808. The small round
symbols were meant to
resemble atoms.
Atoms Are Composed of
Smaller Particles
• J. J. Thomson (1856–1940), an English physicist,
researched the idea that atoms might be made from
a combination of particles. He experimented with
electric currents in glass tubes called cathode ray
tubes.
J. J. Thomson (1856–1940),
• He found that the atom is not the smallest
particle. There were particles within the
atom.
• He theorized that an atom was a
positively charged sphere with negative
charges embedded in it.
J. J. Thomson (1856–1940),
• In 1897 he proposed a
revolutionary new
model for atoms. It is
known as the raisin
bun model. The dough
would be the positively
charged sphere and the
raisins would be the
negative charges.
It is also known as the
plum pudding model
• This would leave the atom with a neutral charge
as the positive charges would be balanced by
the negative charges.
• The negative charges are known as
electrons.
• Electron – Subatomic particle with a
negative charge